Thermomagnetic properties at the nanoscale

Resum:

This Thesis deals with the magnetic properties and magnetocaloric effect (MCE) in the single-domain
range. The motivation to carry out such work is based on the unusual magnetic properties that common
materials exhibit in nanoscaled dimensions when they reach the single-domain size, often radically
different and/or enhanced with respect to their bulk counterparts. These new properties have a wide
range of technological applications, ranging from magnetic recording to biomedicine. In particular, the
study of the MCE in these low-dimensional systems is of primordial importance both for refrigeration
purposes of micro- and nano-electro mechanical systems, and for biomedical applications as magnetic
agents for hyperthermia treatments.
Characterizing the magnetic properties (and the MCE) in these reduced dimensions is very complex,
since the magnetic response of the system is strongly dependent in several factors as size, shape,
anisotropy, dipole-dipole interaction, etc, which make difficult to control the parameters ruling its behaviour,
and consequently, limit their technological use. Furthermore, single-domain magnetic systems
may exhibit superparamagnetic (SPM) behaviour depending on the specific conditions (applied magnetic
field, temperature, magnetic anisotropy, size, shape, etc). SPM behaviour is the paramagnetic-like
temperature dependence that single domain magnetic entities may exhibit at certain conditions, and it
is evident that needs to be perfectly controlled depending on the specific applications we are interested
in (for example, for magnetic recording purposes it is necessary to avoid SPM fluctuations, so that the
magnetic information remains stable against thermal fluctuations).
In this context, the use of a computational technique (Monte Carlo one in our case) arises as a
very useful tool to study such magnetic nanostructures: on the one hand, with a MC method the
characteristics of the system are perfectly controlled and, on the other hand, we can study problems
with no analytical solution, as for example the magnetic dipole-dipole interaction. This is the main
objective of the present work: with the help of a MC technique we can study different nanostructured
systems, as randomly distributed nanoparticles systems or chain-like nanoparticle assemblies, and to
investigate how the different parameter (magnetic anisotropy, size, shape, interparticle interactions,
etc) rule its behaviour. This knowledge will then be applied to search for the optimizing MCE-based
applications both for hyperthermia and refrigeration purposes.